Magnetic field effects on the pyrene-dicyanobenzene system: determination of electron self-exchange rates by MARY spectroscopy

An experimental determination has been made of electron self-exchange rates between the radical anions of 1,2-, 1, 3- and 1,4-dicyanobenzene (DCB) and the respective neutral molecules applying steady-state field-modulated MARY (magnetic field effect on reaction yield) spectroscopy. For the first time this has been achieved successfully for compounds whose self-exchange rate constants can be obtained independently by alternative methods such as EPR linebroadening. In this study, pyrene was used as an electron donor to generate the spin-correlated radical ion pair (pyrene. DCB.) essential for MARY spectroscopy. The radical ion pair is in equilibrium with an exciplex whose magnetic field affected fluorescence was recorded as a function of the magnetic field to yield the MARY spectrum. Due to lifetime uncertainty energy broadening of spin levels caused by electron self-exchange, the characteristic B-1/2 value increases with the concentration of DCB in the sample. The rate constant of self-exchange was obtained from the slope of the linear part in the plot of B-1/2 versus DCB concentration. The values range between 6 x 10(8) M-1 s(1) and 1.4 x 10 M-1 s(1), depending on the DCB isomer and solvent. Comparison with literature data from EPR linebroadening measurements shows good agreement.